Suspensions of polymer-grafted nanoparticles with added polymers—Structure and effective pair-interactions

Abstract

We present the results of combined experimental and theoretical (molecular dynamics simulations and integral equation theory) studies of the structure and effective interactions of suspensions of polymer grafted nanoparticles (PGNPs) in the presence of linear polymers. Due to the absence of systematic experimental and theoretical studies of PGNPs, it is widely believed that the structure and effective interactions in such binary mixtures would be very similar to those of an analogous soft colloidal material-star polymers. In our study, polystyrene-grafted gold nanoparticles with functionality f = 70 were mixed with linear polystyrene (PS) of two different molecular weights for obtaining two PGNP:PS size ratios, ξ = 0.14 and 2.76 (where, ξ = Mg/Mm, Mg and Mm being the molecular weights of grafting and matrix polymers, respectively). The experimental structure factor of PGNPs could be modeled with an effective potential (Model-X), which has been found to be widely applicable for star polymers. Similarly, the structure factor of the blends with ξ = 0.14 could be modeled reasonably well, while the structure of blends with ξ = 2.76 could not be captured, especially for high density of added polymers. A model (Model-Y) for effective interactions between PGNPs in a melt of matrix polymers also failed to provide good agreement with the experimental data for samples with ξ = 2.76 and high density of added polymers. We tentatively attribute this anomaly in modeling the structure factor of blends with ξ = 2.76 to the questionable assumption of Model-X in describing the added polymers as star polymers with functionality 2, which gets manifested in both polymer-polymer and polymer-PGNP interactions especially at higher fractions of added polymers. The failure of Model-Y may be due to the neglect of possible many-body interactions among PGNPs mediated by matrix polymers when the fraction of added polymers is high. These observations point to the need for a new framework to understand not only the structural behavior of PGNPs but also possibly their dynamics and thermo-mechanical properties as well.

Structure of Polymer-Grafted Nanoparticle Melts

“…Instead, one often encounters an undesired aggregation of the (inorganic) NPs in the (organic) polymer matrix, which can lead to a deterioration of material properties and mechanical stability of the composite. Such phase separation can be prevented by grafting the polymers to the surface of the NPs, that is, turning these two component systems into effectively one component systems. − These hybrid particles, known as polymer-grafted NPs (GNPs) or hairy NPs, typically exhibit a core–shell morphology with a hard core and a soft corona. While many previous theoretical descriptions focused on GNPs mixed with small molecule solvents, , we are interested in melts of GNPs where the surrounding medium is composed of other GNPs.…”

Coherent X-ray scattering reveals nature of dynamical transitions in nanoparticle–polymer suspensions

“…These singularities, in star polymers is observed due to the low size asymmetry (size ratios of the two components is closer to 1) as is the case for our mixtures with x ¼ 0.14 (lz1 refer Table 2). It is interesting to note that the behavior of PGNPs, especially in solution and for such large grafting density and grafted chain molecular weight, is likely to be very close to that of star polymers as has been suggested earlier [19,21,37,60,61]. In the binary mixtures of star polymers for lz2, a transition from asymmetric to double glass type is observed (in simulations) through logarithmically decaying correlators.…”

“…While a large volume of data exists for PNCs, equivalent studies on PGNP-polymer binary suspensions are clearly lacking except for few recent reports [21,37,38]. We have recently [21] observed intriguingly similar phase behavior for systems with different x values in the suspensions through the combination of small angle x-ray scattering, coarse grained molecular dynamic simulations and integral equation theory. In addition, we have also observed anomalous dynamic behaviors for PGNP polymer mixtures in both melt and solutions [19,20,37,39].…”

“…The differences in phase behavior with different x values were captured by the variation in the glass transition temperature and the viscosity of the mixtures [25,28]. While a large volume of data exists for PNCs, equivalent studies on PGNP-polymer binary suspensions are clearly lacking except for few recent reports [21,37,38]. We have recently [21] observed intriguingly similar phase behavior for systems with different x values in the suspensions through the combination of small angle x-ray scattering, coarse grained molecular dynamic simulations and integral equation theory.…”

Structure of the Lennard-Jones liquid estimated from a single simulation

“…The structure of an equilibrium liquid is characterized by the radial distribution function g(r). This quantity can be obtained by light scattering experiments, simulation or liquid state theory [1][2][3]. This quantity provides not only an idea of the structure, but also facilitates in predicting the various thermodynamics quantities as g(r) is related to the latter through interparticle interactions [4].…”